The invention relates to a method of chirp pulse echo ranging comprising emitting a long pulse of wave energy characterised for the duration thereof by an oscillation whose frequency is caused to vary linearly and monotonically with time, receiving an echo signal resulting from reflection of the emitted energy pulse from a reflecting object, frequency compressing the received echo signal to form a corresponding echo signal pulse whose duration is short relative to that of the long pulse, and measuring the time delay between a time reference relating to the emission of the long pulse and the occurrence of the pulse of short duration thereby to determine the distance of the reflecting object.
The invention further relates to chirp pulse echo ranging apparatus comprising transmitting means for emitting extended pulses of wave energy each pulse being characterised for the duration thereof by an oscillation whose frequency is varied linearly and monotonically, receiving means for receiving echo signals resulting from reflection of the emitted wave energy from a reflecting object, frequency compression means having a frequency dispersion characteristic such that components of a given applied received echo signal associated with different frequencies of said oscillation are applied to an output of the frequency compression means at substantially the same time to form an echo signal pulse of short duration, and timing means for determining the time delay of the occurrence of the short echo signal pulse relative to a transmitter timing reference pulse thereby to indicate the range of the reflecting object.
The principle of pulse echo ranging is to emit an intense pulse of energy, which can be acoustic (e.g. sonar) or electromagnetic in the form of optical radiation or radio waves (e.g. radar), and to measure the time interval to the subsequent arrival of a reflected echo. The distance to the source of reflection is then derived from the propagation velocity of the energy waves and the measured time for the round trip. The accuracy of the measurement of the range of a reflecting source and the ability to distinguish between echos from different reflection sources which are closely adjacent in range, will depend, inter alia, on the shortness of the received echo pulse on which the timing measurement is performed. However, the range from which a given detectable echo can be reliably received will depend on the fourth power of the emitted energy. Thus when the emitted pulse is made shorter in the interests of accuracy and range discrimination, the peak emitted power has to be increased accordingly to maintain the same effective maximum range. However, the scope for increasing the peak power in the simple pulse arrangement will be limited by factors such as cost and electrical breakdown, saturation of the propagation medium, or cavitation (sonar), and in the case of laser light the safety limit below which a directed beam would not damage a person's eyesight.
Chirp echo ranging method and apparatus of the kind hereinbefore specified, have been developed in order to meet this difficulty by increasing the effective emitted energy while maintaining the accuracy of range determination and discrimination. In the chirp arrangement the emitted energy pulse is lengthened, e.g. by 20 or more times, and is encoded, suitably by means of an oscillation having a monotonically varying frequency, typically a frequency varying linearly with time. The oscillation can comprise the oscillating energy wave forming the emitted pulse.
The frequency compression means comprises in effect a dispersive delay means, and applies a delay to the received echo signal which is greatest for the first part of the signal associated with the initial frequency of the encoded oscillation, and progressively decreases as the oscillation frequency varies monotonically with time until the shortest delay is applied to the last part of the signal associated with the final frequency of the oscillation. The difference between the delays applied to the initial and final oscillation frequency components is made equal to the duration of the emitted pulse with the result that all the oscillation frequency components of a received echo signal arrive at the output of the frequency compressor at the same time thus generating a short echo signal pulse whose time of occurrence can be accurately determined. The shortness of the generated pulse will depend on the receiver bandwidth. It should be understood that the time interval measured between the occurrence of a transmitter timing reference pulse coincident for example with the start of the emitted pulse, and the occurrence of a short echo signal pulse will include not only the time taken for the emitted energy to propagate to and to return from a source of reflection and which is therefore representative of the range thereof, but also the delays introduced by the frequency compressor and other parts of the system which latter are constant or predictable and can be readily taken into account when determining the range from the measured time interval.
Pulse echo ranging systems are frequently required to be able to sense whether a source of reflection has a velocity along the line of sight relative to the ranging apparatus and, if so, to determine the magnitude thereof. In the simple pulse echo system, the line of sight velocity can readily be determined by detecting and measuring the doppler change in the frequency of the returned echo signal. In the case of a chirp echo signal, however, the imposed frequency modulation and long emitted pulse duration introduce difficulties into the measurement of doppler shift.
A CO2 laser rangefinder using heterodyne detection and chirp pulse compression is described by K. F. Hulme et al in Optical and Quantum Electronics, Vol. 13(1981) pages 34-45, which measures the velocity of a target by employing a succession of chirp pulses some of which rise in frequency and others fall in frequency and which are called up-chirp and down-chirp pulses, respectively. If the target is stationary, echos from both the up-chirp and down-chirp transmitted pulses will be output from the chirp decoder at the same time and this will give the true range. If the target is moving along the line of sight, one decoded echo will occur before the correct time while the other will occur after by the same time difference. The actual range can thus be determined from the mean value of the two times of occurrence while the velocity can be determined from the difference. This may be satisfactory in an ideal situation when there is only one object within a given range span to provide a significant echo. In practice, however, the situation will of often occur in which several proximate target objects may be present within a region of interest travelling at different line of sight velocities and it will then become difficult if not impossible to identify the correct pairs of up- and down-chirp echos relating to any- selected target.